Polarization distortion loss (PDL) in fiber communication is caused by random polarization rotation combined with non isotropic loss of fiber (different loss for different polarization).
This loss especially in dual polarization (DP) communication can result in different signal level between the polarization signal in the receiver or in different SNR level.
Long-haul optical communications link typically consists of a number of spans separated by signal recovery units. A model for each span is shown in FIG. 1. The impairments consist of CD (Chromatic Dispersion), PMD (Polarization Mode Dispersion) and PDL (Polarization Dependent Loss). In general, a margin is taken (few dBs depending on the length of the fiber) to compensate for PDL.
The PDL is mostly caused by the EDFA. It is expected that for each EDFA (in each span), one polarity is attenuated more than the other. The attenuation is described in the matrix
      [                                        k            i                                    0                                      0                          1                      ]    ,where ki may be in the order of 0.1-0.2 dB.
After attenuation, the signal is rotated and dispersed. The rotation is described in the matrix
      [                                        cos            ⁢                                                  ⁢                          (                              α                i                            )                                                            sin            ⁡                          (                              α                i                            )                                                                        sin            ⁡                          (                              α                i                            )                                                            cos            ⁡                          (                              α                i                            )                                            ]    ,see FIG. 1, where αi is the rotation angle.
After each span the output in each polarity may be composed of both the input polarity depending on αi, which is a random variable.
So after N spans, the attenuation of one polarity compared to the other is a random variable. The attenuation caused by the PDL has influence on the SNR of each polarity. One polarity has a better SNR than the other polarity.
U.S. Pat. No. 6,760,149 describes a feedback system for PDL compensation. The system includes a monitor, a signal processor, a controller and a compensator. The monitor performs measurements of the transmission parameters. The signal processor calculates the PDL error function and the controller controls the PDL compensator.
There is a need for a system and method of PDL compensation which does not involve feedback and signal processing.
U.S. Pat. No. 6,437,892 discloses PDL compensation in an optical link transmitting light in one polarization state. The system cannot operate with dual-polarization transmission.
There is a need for a system and method of PDL compensation in an optical link with data transmission in dual polarization light, which allows doubling the transmission capacity.
In the preferred embodiment the optical system uses an Orthogonal frequency division multiplexing (OFDM) format. OFDM is widely used technique of transmission in the RF domain where it allows mitigating of signal fading in multi-path propagation. The present invention discloses the use of orthogonal frequency division multiplexing in optical links and, in particular, in fiber communications using dual-polarization transmission.
In optical OFDM each channel the optical carrier is directly modulated by a complex RF signal that can be construed as a linear combination of M separate digitally modulated RF signals at frequencies fm such that fm=m/T where T is the period of modulation. Thus the total symbol rate of the transmitted information is M/T. In the text we shall refer to the frequencies fm as “sub-carriers”.
In modern communication systems, a coherent detection technique is implemented, which provides improved sensitivity compared with traditional direct detection schematics. Typically coherent detection is used for phase-shift-keying (PSK) data transmission. The present invention is also focused on PSK, and in the preferred embodiment, QPSK (quadrature PSK) data transmission. However this does not limit the scope of the invention, and various types of data modulation can benefit from the disclosed invention.
In coherent receiver, the QPSK incoming optical signal is mixed with a strong local oscillator signal to produce in-phase (I) and in-quadrature (Q) outputs. I and Q components of the output optical signal are converted into electrical signals by a set of photodetectors. In the preferred configuration four balanced photodetectors are used to recover QPSK encoded data.
Data transmission using light of two orthogonal polarizations via the same optical channel allows doubling the data rate. At the receiver side, the optical signal is split by a polarization beam splitter, and the light of each orthogonal polarization is mixed with a local oscillator signal of the corresponding polarization in the coherent receiver.
However the orthogonality of the optical signal polarization is not preserved when the signals are transmitted via fiber link. The received optical signals neither orthogonal to each other, nor aligned with the polarization beam splitter at the receiver side. The present invention addresses this problem of the polarization state recovery in dual-polarization data transmission.